<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .bar {
      width: 150px;
      height: 15px;
      border: 1px solid red;
      border-radius: 7px;
    }
    .bar_in {
      width: 50%;
      height: 100%;
      background-color: red;
      transition: all .7s;
    }

    .bar:hover .bar_in {
      width: 100%;
    }
  </style>
</head>
<body>
  <div class="bar">
    <div class="bar_in"></div>
  </div>
</body>
</html>